In Philip Rivers, new Chargers quarterbacks coach Frank Reich inherits a 31-year-old whose quarterback rating has dropped each of the last four years.

Rivers set the bar high before his recent slippage. In 2008, his quarerback rating (105.5) led the league.

Consider that his rating of 88.6 last year, which was 11th in the NFL and his lowest in five years, still beat Joe Flacco’s 87.7.

For a 28-game stretch that began with the 2011 opener, however, Rivers had the most turnovers (43) in the NFL. Bad decisions by Rivers and blocking leaks played a role, as did the offense’s talent drain overall. Ex-QB Steve Young, an ESPN analyst, faulted Rivers’ footwork at times.

Reich, 51, is a former NFL quarterback. The same cannot be said for Rivers’ previous coaches. Reich went to four consecutive Super Bowls with the Bills as the backup to Jim Kelly, learning a lot.

“Frank understands football,” said Ted Tollner, who was a Bills aide for part of Buffalo’s AFC dynasty. “The other thing is, he’s an excellent coach. He deals with people really well.”

To hear Tollner, if Reich assists Rivers to the same degree that he did Kelly, a Hall of Famer, Rivers will improve.

“Jim had a lot of confidence in what Frank saw and what Frank would say to him on the field,” Tollner said. “Frank complemented, I thought, very, very well. He helped Jim as a field guy.”

Peyton Manning is another passer with Hall of Fame credentials whom Reich knows well, from Reich’s time in 2009-10 as the Colts’ quarterbacks coach.

As the Colts’ receivers coach, he tutored Reggie Wayne in 2011. Last year Reich was the receivers coach for the Cardinals. In San Diego, he is reunited with offensive coordinator Ken Whisenhunt, Arizona’s head coach from 2007-12.